Method of making a fine grained cemented carbide

ABSTRACT

According to the present invention there is provided a method of making a finegrained tungsten carbide—cobalt cemented carbide comprising mixing, milling according to standard practice followed by sintering. By introducing nitrogen at a pressure of more than 0.5 atm into the sintering atmosphere after dewaxing but before pore closure a grain refinement including reduced grain size and less abnormal grains can be obtained.

BACKGROUND OF THE INVENTION

The present invention relates to a method of making a fine grained cemented carbide. By performing the sintering at least partly in a nitrogen-containing atmosphere, a grain refined cemented carbide structure has been obtained.

Cemented carbide inserts with a grain refined structure are today used to a great extent for machining of steel, stainless steels and heat resistant alloys in applications with high demands on both toughness and wear resistance. Another important application is in microdrills for the machining of printed circuit board so called PCB-drills.

Common grain growth inhibitors include vanadium, chromium, tantalum, niobium and/or titanium or compounds involving these. When added, generally as carbides, they limit grain growth during sintering, but they also have undesirable side effects such as unfavorably affecting the toughness behavior. Additions of vanadium or chromium are particularly detrimental and have to be kept on a very low level in order to limit their negative influence on the sintering behavior. Both vanadium and chromium reduce the sintering activity often resulting in an uneven binder phase distribution and toughness reducing defects in the sintered structure. Large additions are also known to result in precipitation of embrittling phases in the WC/Co grain boundaries. According to WO 99/13120, the amount of grain growth inhibitors can be reduced if a carbon content of the cemented carbide close to eta-phase formation is chosen.

In order to maintain a fine grain size, sintering is generally performed at a relatively low temperature of 1360° C. followed by sinterHIP in order to obtain a dense structure. Such production route, of course, increases the production cost.

It is known that tungsten carbonitride can be produced by high pressure nitrogen treatment of a mixture of tungsten and graphite powder. The process is described in JP-A-03-208811 and JP-A-11-35327 and it is claimed that the resulting tungsten carbonitride powder can be used as a raw material for manufacturing of super hard alloys. JP-A-11-152535 discloses a process to manufacture fine grained tungsten carbonitride—cobalt hard alloys using tungsten carbonitride as a raw material.

JP-A-10-324942 and JP-A-10-324943 disclose methods to produce ultra-fine grained cemented carbide by adding the grain growth inhibitors as nitrides. In order to avoid pore formation by denitrification of the nitrides sintering is performed in a nitrogen atmosphere.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to avoid or alleviate the problems of the prior art. It is further an object of the present invention to provide a cemented carbide insert with a combination of high toughness and high deformation resistance along with a method for making the same.

There is provided in a method of making a finegrained tungsten carbide—cobalt cemented carbide comprising mixing, milling and sintering, the improvement comprising introducing-nitrogen at a pressure of more than about 0.5 atm into the sintering atmosphere after dewaxing but before pore closure.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 shows in about 1500× a typical example of the structure of a “pure” WC-Co grade, alloyed with nitrogen by sintering according to the invention.

FIG. 2 shows in about 1500× a typical example of the structure of the same grade sintered according to prior art.

FIG. 3 shows in about 1500× a typical example of the structure of the same grade, alloyed with nitrogen by sintering according to the invention, after sintering at reduced temperature.

FIG. 4 shows in about 1500× a typical example of the structure after conventional sintering at reduced temperature.

FIG. 5 shows in about 1200× a typical example of the structure of the Cr₃C₂ containing WC-Co grade, alloyed with nitrogen by sintering according to the invention, after sintering at reduced temperature.

FIG. 6 shows in about 1200× a typical example of the structure of the same grade after conventional sintering at reduced temperature.

FIG. 7 shows in about 1200× a typical example of the structure of a “pure” submicron (0.25 μm) WC-Co grade, alloyed with nitrogen by sintering according to the invention.

FIG. 8 shows in about 1200× a typical example of the structure of the same grade sintered according to prior art.

FIG. 9 shows in about 1200× a typical example of the structure of a Cr₃C₂ containing submicron 0.25 μm WC-Co grade, alloyed with nitrogen by sintering according to the invention.

FIG. 10 shows in about 1200× a typical example of the structure of the same grade after conventional sintering.

FIG. 11 shows in about 1200× a typical example of the structure of a Cr₃C₂ containing submicron 0.6 μm WC-Co grade, alloyed with nitrogen by sintering according to the invention.

FIG. 12 shows in about 1200× a typical example of the structure of the same grade after conventional sintering.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

It has now surprisingly been found that a pronounced grain refining effect in combination with an improved binder phase distribution can be obtained by introduction of nitrogen as a process gas in sintering furnace prior to pore closure.

The method according to the present invention comprises mixing, milling and pressing of tungsten carbide—cobalt bodies according to conventional powder metallurgical methods followed by sintering in a process characterized by the introduction of nitrogen at a pressure of more than 0.5 atm, preferably more than 0.75 atm, into the sintering atmosphere after dewaxing but before pore closure, preferably before 1000° C.

In one embodiment, the whole sintering process is performed in nitrogen.

In an alternative embodiment, the nitrogen is after pore closure replaced by a protective atmosphere of e.g. argon or vacuum.

The resulting sintered body is characterized by a grain refined structure, reduced grain size and less abnormal grains, in combination with an improved binder phase distribution compared to sintering according to normal practices, and with a nitrogen content of more than 0.03 weight-%, preferably more than 0.05 weight-%.

The cobalt content for these alloys should be in the range 4 to 15 weight-%, preferably 5 to 12 weight-%.

The average number of abnormal grains can be determined using inserts etched for 2 minutes at room temperature in Murakamis regent, examining the etched surface with optical microscope at 1500×, counting the number of abnormal grains on ten micrographs, taken randomly from the surface, and calculating the average number of abnormal grains per micrograph. Each micrograph corresponds to a surface area of 8360 μm².

Using the process of the invention with a pure WC-Co alloy, the average number of abnormal grains per micrograph, having a maximum length in any direction >15 μm, is <1.0, preferably <0.7. The average number of abnormal grains per micrograph, having a maximum length in any direction >20 μm, is <0.5. The average number of abnormal grains per micrograph, having a maximum length in any direction >25 μm, is <0.1.

Using the process of the invention with a WC-Co alloy containing grain growth inhibitors the average number of abnormal grains per micrograph, having a maximum length in any direction >5 μm, is <0.15.

For a WC grain size below 0.5 μm, the beneficial effect of nitrogen alloying has to be combined with an addition of conventional grain growth inhibitors from groups IVb, Vb and/or VIb of the periodic table, preferably Cr, V and/or Ta, most preferably Cr and/or Ta, either as pure metals or compounds thereof except the nitrides thereof, preferably compounds free of nitrogen, most preferably carbides.

The process of the invention works on pure WC-Co alloys as well as on WC-Co alloys containing grain growth inhibitors. But the most significant improvement regarding grain growth control has been seen for straight WC-Co alloys with a sintered average grain size of <1.5 μm, preferably <1 μm but larger than 0.5 μm where no further grain growth inhibitors are necessary.

It has thus been found that the introduction of nitrogen into the sintering furnace after dewaxing but before pore closure results in a significant nitrogen pickup even for nominally pure WC-Co alloys. It has further surprisingly been found that the introduced nitrogen acts as a grain growth inhibitor at the same time as it improves the sintering activity and thus the resulting binder phase distribution. It has also been found that the nitrogen content achieved before pore closure becomes entrapped as soon as the temperature becomes high enough for pore closure. Extended sintering time in vacuum after pore closure has been found to have only a minor effect on the resulting nitrogen content in the as sintered samples.

The invention is additionally illustrated in connection with the following Examples, which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the Examples.

EXAMPLE 1

From a powder mixture consisting of 6.0 weight-% Co, and balance WC with an average grain size of about 1 μm with 0.01 weight-% overstoichiometric carbon content, turning the inserts CNMG120408 were pressed. The inserts were sintered in H₂ up to 450° C. for dewaxing. At 450° C., the furnace was evacuated and refilled with nitrogen up to a pressure of 0.8 atm. The temperature was kept constant at 450° C. during the nitrogen filling procedure. After completed filling, the temperature was increased to 1370° C. with a speed of 15° C./min, keeping the nitrogen pressure constant. At 1370° C., the furnace was evacuated and refilled with a protective atmosphere of 10 mbar Argon and kept at 1370° C. for 30 minutes followed by an increased Ar pressure of 40 mbar and a temperature increase up to the final sintering temperature of 1410° C. where the temperature was kept for an additional hour before cooling and opening of the furnace.

The structure in the cutting inserts consisted of comparably fine and uniform tungsten carbide grain size in combination with a good binder phase distribution, FIG. 1.

EXAMPLE 2 Reference Example to Example 1

Pressed inserts from Example 1 were sintered in H₂ up to 450° C. for dewaxing, further in vacuum to 1370° C., then filled with a protective gas of 10 mbar of Ar and kept at 1370° C. for 30 minutes followed by an increased Ar pressure of 40 mbar and a temperature increase up to the final sintering temperature of 1410° C. where the temperature was kept for an additional hour before cooling and opening of the furnace.

The structure in the cutting inserts consisted of a comparably less fine and uniform tungsten carbide grain size in combination with a acceptable binder phase distribution, FIG. 2.

EXAMPLE 3

Pressed inserts from Example 1 were sintered in H₂ up to 450° C. for dewaxing. At 450° C., the furnace was evacuated and refilled with nitrogen up to a pressure of 0.8 atm. The temperature was kept constant at 450° C. during the nitrogen filling procedure. After completed filling, the temperature was increased to 1370° C. with a speed of 15° C./min, keeping the nitrogen pressure constant. At 1370° C. the furnace was evacuated and refilled with a protective atmosphere of 10 mbar Argon. The actual sintering was limited to a 30 min hold at 1370° C. followed by cooling and opening of the furnace.

The structure in the cutting inserts consisted of comparably fine and uniform tungsten carbide grain size in combination with an acceptable binder phase distribution, FIG. 3.

EXAMPLE 4 Reference Example to Example 3

Pressed inserts from Example 1 were sintered in H₂ up to 450° C. for dewaxing and further in vacuum to 1370° C. At 1370° C., the furnace was filled with a protective atmosphere of 10 mbar Argon. The actual sintering was limited to a 30 min hold at 1370° C. followed by cooling and opening of the furnace.

The structure in the cutting inserts consisted of a comparably less fine and uniform tungsten carbide grain size in combination with an unacceptable binder phase distribution, FIG. 4.

EXAMPLE 5

From a powder mixture consisting of 5.2 weight-% Co, 0.6 weight-% Cr₃C₂ and balance WC with an average grain size of about 1 μm with 0.05 weight-% overstoichiometric carbon content, turning inserts CNMG120408 were pressed. The inserts were sintered in H₂ up to 450° C. for dewaxing. At 450° C., the furnace was evacuated and refilled with nitrogen up to a pressure of 0.8 atm. The temperature was kept constant at 450° C. during the nitrogen filling procedure. After completed filling, the temperature was increased to 1370° C. with a speed of 15° C./min, keeping the nitrogen pressure constant. At 1370° C. the furnace was evacuated and refilled with a protective atmosphere of 10 mbar Argon. The actual sintering was limited to a 30 min hold at 1370° C. followed by cooling and opening of the furnace.

The structure in the cutting inserts consisted of comparably fine and uniform tungsten carbide grain size in combination with a good binder phase distribution, FIG. 5.

EXAMPLE 6 Reference Example to Example 5

Pressed inserts from Example 5 were sintered in H₂ up to 450° C. for dewaxing, further in vacuum to 1370° C. At 1370 the furnace was filled with a protective atmosphere of 10 mbar Argon. The actual sintering was limited to a 30 min hold at 1370° C. followed by cooling and opening of the furnace.

The structure in the cutting inserts consisted of a comparably less fine and uniform tungsten carbide grain size in combination with an unacceptable binder phase distribution, FIG. 6.

EXAMPLE 7

From a powder mixture consisting of 10.0 weight-% Co, and balance WC with an average grain size of about 0.25 μm with 0.01 weight-% overstoichiometric carbon content, turning inserts CNMG120408 were pressed. The inserts were sintered in H₂ up to 450° C. for dewaxing. At 450° C., the furnace was evacuated and refilled with nitrogen up to a pressure of 0.8 atm. The temperature was kept constant at 450° C. during the nitrogen filling procedure. After completed filling, the temperature was increased to 1370° C. with a speed of 15° C./min, keeping the nitrogen pressure constant. At 1370° C., the furnace was evacuated and refilled with a protective atmosphere of 10 mbar Argon and kept at 1370° C. for 30 minutes followed by an increased Ar pressure of 40 mbar and a temperature increase up to the final sintering temperature of 1410° C. where the temperature was kept for an additional hour before cooling and opening of the furnace.

The structure in the cutting inserts consisted of compared to the reference in example 8 finer large tungsten carbide grains in combination with a good binder phase distribution, FIG. 7.

EXAMPLE 8 Reference Example to EXAMPLE 7

Pressed inserts from Example 7 were sintered in H₂ up to 450° C. for dewaxing, further in vacuum to 1370° C., then filled with an protective gas of 10 mbar of Ar and kept at 1370° C. for 30 minutes followed by an increased Ar pressure of 40 mbar and a temperature increase up to the final sintering temperature of 1410° C. where the temperature was kept for an additional hour before cooling and opening of the furnace.

The structure in the cutting inserts consisted of large grains and a non-uniform tungsten carbide grain size in combination with an acceptable binder phase distribution, FIG. 8.

EXAMPLE 9

Inserts from Example 7 and 8 were etched for 2 minutes at room temperature in Murakamis regent and examined under optical microscope at 1500×. Ten micrographs were taken. In all ten micrographs, WC grains having a length in any direction >15 μm were detected and the maximum length for each such grain was measured. An average number of abnormal grains per micrograph, corresponding to a surface area of 8360 μm², was calculated by dividing the number of grains by 10.

Result: Average number of grains with max. length >15 μm >20 μm >25 μm Example 7 0.33 0 0 (invention) Example 8 1.4 0.6 0.2 (reference)

EXAMPLE 10

From a powder mixture consisting of 10.0 weight-% Co, 0.5 weight-% Cr₃C₂ and balance WC with an average grain size of about 0.25 μm with 0.05 weight-% overstoichiometric carbon content, turning inserts SNUN were pressed. The inserts were sintered in H₂ up to 450° C. for dewaxing. At 450° C. the furnace was evacuated and refilled with nitrogen up to a pressure of 0.8 atm. The temperature was kept constant at 450° C. during the nitrogen filling procedure. After completed filling, the temperature was increased to 1370° C. with a speed of 15° C./min, keeping the nitrogen pressure constant. At 1370° C., the furnace was evacuated and refilled with a protective atmosphere of 10 mbar Argon and kept at 1370° C. for 30 minutes followed by an increased Ar pressure of 40 mbar and a temperature increase up to the final sintering temperature of 1410° C. where the temperature was kept for an additional hour before cooling and opening of the furnace.

The structure of the cutting inserts consisted of a uniform submicron tungsten carbide grain size in combination with an almost absence of large grains and a uniform Co distribution, FIG. 9.

EXAMPLE 11 Reference Example to EXAMPLE 10

Pressed inserts from Example 10 were sintered in H2 up to 450° C. for dewaxing, further in vacuum to 1370° C., then filled with a protective gas of 10 mbar of Ar and kept at 1370° C. for 30 minutes followed by an increased Ar pressure of 40 mbar and a temperature increase up to the final sintering temperature 1410° C. where the temperature was kept for an additional hour before cooling and opening of the furnace.

The structure in the cutting inserts consisted of a less uniform submicron tungsten carbide grain size and in combination with some large WC grains, FIG. 10.

EXAMPLE 12

Inserts from Example 10 and 11 were etched for 2 minutes at room temperature in Murakamis regent and examined under optical microscope at 1500×. Ten micrographs were taken. In all ten micrographs, WC grains having a length in any direction >5 μm were detected and the maximum length for each such grain was measured. An average number of abnormal grains per micrograph, corresponding to a surface area of 8360 μm², was calculated by dividing the number of grains by 10.

Result: Average number of grains with max. length >5 μm Example 10 (invention)   0-0.1 Example 11 (reference) 0.25-0.4

EXAMPLE 13

From a powder mixture consisting of 10.0 weight-% Co, 0.5 weight-% Cr₃C₂ and balance WC with an average grain size of about 0.6 μm with 0.05 weight-% overstoichiometric carbon content, turning inserts SNUN were pressed. The inserts were sintered in H₂ up to 450° C. for dewaxing. At 450° C., the furnace was evacuated and refilled with nitrogen up to a pressure of 0.8 atm. The temperature was kept constant at 450° C. during the nitrogen filling procedure. After completed filling, the temperature was increased to 1370° C. with a speed of 15° C./min, keeping the nitrogen pressure constant. At 1370° C., the furnace was evacuated and refilled with a protective atmosphere of 10 mbar Argon and kept at 1370° C. for 30 minutes followed by an increased Ar pressure of 40 mbar and a temperature increase up to the final sintering temperature of 1410° C. where the temperature was kept for an additional hour before cooling and opening of the furnace.

The structure in the cutting inserts consisted of a uniform submicron tungsten carbide grain size and in combination with an almost absence of large grains and a uniform Co distribution, FIG. 11.

EXAMPLE 14 Reference Example to EXAMPLE 13

Pressed inserts from Example 13 were sintered in H₂ up to 450° C. for dewaxing, further in vacuum to 1370° C., then filled with a protective gas of 10 mbar of Ar and kept at 1370° C. for 30 minutes followed by an increased Ar pressure of 40 mbar and a temperature increase up to the final sintering temperature 1410° C. where the temperature was kept for an additional hour before cooling and opening of the furnace.

The structure in the cutting inserts consisted of a less uniform submicron tungsten carbide grain size and in combination with some large WC grains, FIG. 12.

EXAMPLE 15

Inserts from Example 13 and 14 were etched for 2 minutes at room temperature in Murakamis regent and examined under optical microscope at 1500×. Ten micrographs were taken. In all ten micrographs, WC grains having a length in any direction >5 μm were detected and the maximum length for each such grain was measured. An average per micrograph was calculated by dividing the number of grains by 10. An average number of abnormal grains per micrograph, corresponding to a surface area of 8360 μm², was calculated by dividing the number of grains by 10.

Result: Average number of grains with max. length >5 μm Example 13 (invention)   0-0.1 Example 14 (reference) 0.2-0.4

The principles, preferred embodiments, and modes of operation of the present invention have been described in the foregoing specification. The invention, which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention. 

1. In a method of making a finegrained tungsten carbide—cobalt cemented carbide comprising mixing, milling and sintering, the improvement comprising introducing nitrogen at a pressure of more than about 0.5 atm into the sintering atmosphere after dewaxing but before pore closure.
 2. In the method of claim 1 wherein nitrogen is introduced at a pressure more than about 0.75 atm.
 3. In the method of claim 1 wherein the nitrogen is introduced into the sintering process before the sintering temperature reaches about 1000° C.
 4. In the method of claim 1 wherein the entire sintering process is performed in nitrogen.
 5. In the method of claim 1 wherein the nitrogen after pore closure is replaced by a protective atmosphere.
 6. In the method of claim 5 wherein the protective atmosphere is argon or a vacuum.
 7. In the method of claim 1 wherein grain growth inhibitors of metals from groups IVb, Vb and/or VIb of the periodic table or compounds thereof except nitrides are added to the mixture.
 8. In the method of claim 7 wherein the grain growth inhibitors are metals or compounds of Cr, V and/or Ta.
 9. In the method of claim 8, wherein the grain growth inhibitors are metals or compounds of Cr and/or Ta.
 10. In the method of claim 7, wherein said compounds are free of nitrogen.
 11. In the method of claim 7, wherein said compounds are carbides. 